This invention relates to a method for increasing the density of cross-linked haloalkylated monovinylidene aromatic polymers and to methods for preparing high density ion exchange resins therefrom.
Ion exchange resins are normally solid materials which generally carry exchangeable ions. Due to their ability to exchange ions in a liquid without substantial alteration of the solid resin's structure, they are widely used in recovery processes such as uranium recovery and in waste treatment such as the removal of undersirable components from water.
Generally, the most effective ion exchange resins are substantially insoluble but swellable to a limited degree in water and are resistant to physical deterioration such as excessive swelling or shattering. Moreover, in many applications, particularly when employed in continuous operations such as uranium recovery or sugar processing, the resin's density is advantageously sufficiently high to assure initimate and continuous contact between the resin and the ion containing liquids and to sink the resin in said liquids, which are generally thick slurries or pulps.
Conventionally, many ion exchange resins are prepared by (1) haloalkylating a copolymer of (a) a monovinylidene aromatic such as styrene and (b) a cross-linking agent which is generally a polyvinylidene aromatic such as divinylbenzene in the presence of a Friedel-Crafts catalyst and (2) attaching ion active exchange groups to the resulting haloalkylated polymer. For example, an anion exchange resin is prepared by aminating the haloalkylated polymer. See, Ion Exchange by F. Helfferich, published in 1962 by McGraw-Hill Book Company, New York. Unfortunately, these anion exchange resins, without modification, possess relatively low densities, thereby tending to float in thicker slurries or pulps.
Methods for increasing the density of such resins are known in the art. For example, U.S. Pat. Nos. 2,769,788 and 2,809,943 disclose methods for incorporating inert, finely divided solid materials having a high density, i.e., 2.5 g/cc or higher, into copolymer beads of monovinylidene and polyvinylidene aromatic compounds. Unfortunately, the ion exchange resin beads prepared by such methods exhibit surface irregularities, excessive spalling, and low mechanical stability, thereby, tending to break when employed in a continuous operation.
To increase the mechanical stability of high density ion exchange resin beads, German Pat. No. 2,218,126 proposes preparing the resins using a non-ionic substituted styrene, such as monochlorostyrene, as the monovinylidene aromatic compound. Unfortunately, non-ionic substituted styrenes are relatively expensive and of limited availability.
In view of these stated deficiencies in the prior art methods for preparing ion exchange resins having higher densities, it would be highly desirable to provide an improved method for preparing such resins.